(1) Field of the Invention
The present invention relates to the field of mobile telephony. In particular, the present invention relates to an apparatus and a method for determining the geographic location of a mobile transceiver.
(2) Description of Related Art
Emergency calls placed from someone involved in or observing an emergency situation are easily placed and routed to the appropriate emergency response authorities by dialing the number 911. This system is therefore called the 911 call response system. Because the person placing the call may be under stress, duress, or be otherwise incapacitated, a newer version of the 911 call response system, called Enhanced 911 (E911), is entering service. In the E911 system the location of the caller is sent to the emergency response authorities, enabling a response even when the caller can do little more than dial the number 911. For calls placed from a fixed location, the telephone companies"" billing databases may be searched to give a location associated with each originating telephone number.
The E911 system is more difficult to implement for mobile (cellular) telephones. Cellular telephones by their nature may vary in location. However, at the present time about half of all E911 calls are placed from a cellular phone. For this reason the Federal Communications Commission (FCC) has mandated that a high percentage of cellular E911 calls be able to report the caller""s location within 125 meters.
One method of locating the mobile telephone""s location consists of placing additional equipment into the mobile telephone so that it may independently determine its geographic location. For example, a Global Positioning System (GPS) receiver could be incorporated into the mobile telephone. This method has several drawbacks. The addition of the GPS receiver adds cost and bulk to the mobile receiver. Additionally, the GPS receiver requires clear radio-frequency (RF) access to multiple GPS satellites in order to calculate a position. There are many circumstances where a mobile phone could place a call yet not have RF access to multiple GPS satellites.
Another method of locating the mobile telephone""s location derives the location using only the mobile telephone system""s equipment. One method proposed to allow the temporary location of a cellular telephone to be determined and routed to the emergency response authorities is called reverse link triangulation. Cellular phone connections consist of forward links sent from fixed base stations to a mobile station, and reverse links sent from a mobile station to a fixed base station. In reverse link triangulation, a mobile station broadcasts a special signal that may be received by multiple base stations. If the base stations each have a synchronized local time standard, the differential time of arrival (DTOA) of the special signal at the various base stations may be determined by reading each local time standard when the special signal arrives at each base station. From the DTOA data and the speed of light may be calculated the distance from the mobile station to each base station. If three or more base stations receive the special signal, and the location of these base stations is known, then a computational element in the cellular phone system may determine the location of the mobile station.
Reverse link triangulation advantageously utilizes the existing mobile telephone system. However, it has limitations. The mobile telephone is limited in transmission power. For this reason it is not always possible for the special signal of reverse link triangulation to be received by three or more base stations.
Therefore a method for reliably determining the geographic location of a mobile telephone is at issue in mobile telephone systems.
A system and method for determining the geographic location of a mobile station in a mobile telephone system is disclosed. The method begins with broadcasting a series of forward link triangulation signals from a series of base transceiver stations. The mobile station then receives and timestamps the forward link triangulation signals. By calculating a first hyperbola and a second hyperbola from these timestamped forward link triangulation signals, the intersection of the first hyperbola and second hyperbola will yield the desired geographic location of the mobile station.